Earthquakes induced by fluid injection in geothermal and other energy source production area are becoming important topics because of possible damages they can cause and opportunity to better understand the influence of fluids in the faulting process. However, the nucleation, rupture detail and termination of induced events are poorly known. Here, we show joint analysis of UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar) data, strong motion record, field observations and leveling measurements for the extreme shallow earthquakes in the geothermal field at the center of Imperial Valley basin. We found that the centroid depths of these normal faulting are as shallow as 2.0km associated with unmapped surface breakage. The joint inversion of geodetic and seismic data for the Mw4.7 event reveals a short risetime (~0.4s) and relative high rupture speed (1.75km/s, 90% of local shear wave speed). The coseismic slip is complementary distributed to the aseismic transient happened 2 years prior to earthquake. The spatial correlation between the location of geothermal injection wells and the subsidence along with coulomb stress analysis from previous swarm activity suggests the possibility of multiple triggering mechanisms for this shallow extensional event.

Intellectual Merit

It is well known that earthquakes can be induced primarily by stress release of human activity or as a result of tectonic stress releasing triggered by such activities. The earthquakes that are generated by industrial activity, such as hydraulic fracturing, itself is easier to discriminate from natural earthquakes since they usually happen right after the injection and co-located with high injection pressure wells. The triggered earthquakes, however, are much harder to distinguish from regular earthquakes, for example, the 2011 Oklahoma earthquake happened about two decades after the disposal of the injection [Keranen et al., 2013]. There are mainly two mechanisms for inducing earthquakes which are increase of pore pressure acting on a fault and by changing the shear and normal stress on the fault. The fluid migration is one of the largest uncertainties behind such mechanisms [Ellsworth, 2013]. To solve the problem for identifying triggered events and better understand the mechanism behind them, it is particularly helpful to study individual triggered events in greater detail. However, because induced or triggered events are usually small (M<4), most of the previous works treated them as point sources. Also due to their limited sizes, researchers mainly rely on nearby seismic data to constrain the location and the focal mechanisms of such earthquakes. To use these waveform data more effectively requires waveform inversion at higher frequency ranges which is not easy due to the complexity of the 3D velocity structure. In short, investigations of finiteness of individual induced events have seldom been conducted but should become a key issue in forensic investigations of shallow events.

Broader Impacts

While great earthquakes are extremely important to understand, swarm events may provide more key information about the rupture process. Because they are sometimes associated with fluid-injection displaying direct evidence for the role of fluid in a man-made experiment, in which surface geodetic data is available for constraining the slip-distribution. Industrial induced earthquakes have become an important topic due to the possible damage and the increase of seismicity they can cause [Ellsworth, 2013; Gonzalez et al., 2012]. In particular, special attention has been paid to fluid injection in geothermal and other energy source production area [Bourouis and Bernard, 2007; Brodsky and Lajoie, 2013]. Before studying the relation between these earthquakes and human activity, the first challenge is to distinguish them from regular tectonic earthquakes. Detailed spatial and temporal pattern of these earthquakes and their mechanisms are very useful for this purpose as discussed above. To monitor ground deformation and search for the unmapped faults in the injection region one can use high-resolution plane based UAVSAR techniques in combination with high-resolution modeling to help understand the aseismic vs seism process.

Exemplary Figure

FIGURE High-resolution finite fault slip models for the largest three events (Mw5.3, Mw5.4, Mw4.9) in the 2012 Brawley earthquake swarm. High frequency (up to 3Hz) waveform inversion is enabled by the path calibration from a smaller (Mw3.9) event. The location and point source mechanisms of these events are shown in (a) along with the seismic and GPS stations (triangles). The red rectangle is the map view of the fault plane used for the strike-slip (Mw5.4, Mw5.3) events. The slip models of the Mw5.3 and Mw5.4 events are shown in (b) and (c), respectively. The black triangle in (b) and (c) denotes the same location on the fault which highlights the complementary slip distribution between the two earthquakes. The Mw4.9 normal earthquake with shallow depth (~2km) has generated ground offsets that were well observed by the UAVSAR data (d). Finite fault inversion shows a coseismic slip dimension of ~3km×3km with average offsets of about 20cm (e). Inversion of the leveling data reveals a bull-eye aseismic slip pattern located on the same fault of the Mw4.9 earthquake, centered at about 2km to the south of the epicenter with similar centroid depth of the earthquake. Note the complementary feature between the seismic and aseismic slips.